My view varies quite a lot depending on the reviewer/review, but generally:
"medium, only talking highlights, and maybe lowlights, too"
The history of the album history I could care less about. If I wanted to hear about it, I'd read a bio of the band. I like a review to be clever, and if the reviewer drones on and on going through the same old formula with seemingly no inspiration, I'll just close it and ignore the review.
I've written one very long and in-depth review for Pendragon's Believe, and I do believe it is just an average review of mine. I explain the songs well, I believe, but usually just an overview and feel of the album is fine. Picking out two or three songs that best represent the album and explaining why they represent the album is just fine.
For me, the ideal review is no less than 100 words but no more than 500, depending on how well the reviewer can connect his thoughts and feelings.